The IEEE 802.11s standardization committee group is currently working on an extension of the 802.11 standard for meshes. The current IEEE 802.11s standard specification, version D1.06, incorporated herein by reference, defines an IEEE 802.11 WLAN using the IEEE 802.11 Medium Access Control/Physical (MAC/PHY) layers that support both individually addressed and group addressed delivery over self-configuring multi-hop topologies. Mesh networks according to the 802.11s standard, or so-called meshes, operate as wireless co-operative communication infrastructures between numerous individual wireless transceivers.
Stations or mesh points (MP) define nodes in the mesh that communicate with their neighboring adjacent nodes only.
MPs thus act as repeaters to transmit message data from nearby nodes to peers that are too far to reach.
More generally, in wireless communication systems, transmissions are vulnerable to collisions as frames may be transmitted simultaneously. Therefore systems are usually built in with a number of preventive measures to reduce the number of collisions.
Examples from the IEEE 802.11 standard include Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Request To Send/Clear To Send (RTS/CTS) virtual carried sense protocols.
In particular, the CSMA/CA protocol is designed for reducing the collision probability between multiple stations accessing a medium at the points where collisions would most likely occur.
To this purpose, and for a station to transmit, the method of the CSMA/CA protocol senses the medium to determine the state of the medium (i.e. if another station is transmitting), by using physical (provided by the PHY) and virtual (provided by the MAC) carrier-sense functions. When either function indicates a busy medium, the medium shall be considered busy; otherwise, it shall be considered idle.
If the medium is determined to be busy, the station shall defer its transmission until the end of the current transmission.
After deferral, or prior to attempting to transmit again immediately after a successful transmission, the station selects a random backoff interval and decrements the backoff interval counter while the medium is idle.
Such a random backoff procedure is necessary to resolve potential transmissions collisions on the medium that could occur after a busy time. There is indeed a high probability of a collision at this time, since multiple stations could have been waiting for the medium to become available during the busy time. Therefore, if several stations compete to access the medium, it is the station having the shortest random time that is allowed to access the medium. The other stations must therefore defer their transmission.
This backoff procedure allows thus a station to randomly own the access to the medium after a random time of time.
Moreover, the backoff procedure suspends the random backoff time whenever the medium becomes busy. It is resumed as soon as the medium is idle again.
If the medium is idle after the random time, the station may then initiate a frame or a sequence of frames exchange. It is to be noticed that a “sequence of frames” is for example built from a fragmentation of an initial MAC data unit into a sequence of smaller MAC level frames separately transmitted or come from independent burst of independent frames.
Additionally, the CSMA/CA protocol ensures that a gap of a minimum specified duration exists between contiguous frames.
To this purpose, the CSMA/CA provides fixed deferral times known as Interframe Spaces (IFSs) between frames or sequences of frames and/or before starting the backoff procedure—see Section 9.2.3. of the IEEE 802.11-2007 standard incorporated herein by reference. The durations of the IFSs are predetermined fixed by the PHY—see Sections 9.2.10 and 9.9.1.3 of the 802.11-2007 standard incorporated herein by reference.
Some examples of IFSs as defined in the IEEE 802.11 standard are given below.
The Short IFS (SIFS) is a fixed time interval from the end of the last symbol of a previous frame to the beginning of the first symbol of the preamble of the subsequent frame. A SIFS is typically used for separating two successive data frames of a sequence of frames, or a received data frame from a RTS, CTS or ACK frame to transmit.
The Distributed coordination function IFS (DIFS) is a fixed time interval that a station waits once the medium is sensed to be idle after a correctly received frame. Once the DIFS elapsed and if the medium is still idle, the random backoff time is decremented.
The Extended IFS (EIFS) starts once the medium is sensed to be idle after a transmitted frame was not received correctly. The EIFS is defined to provide enough time for another station to acknowledge what was, to this station, an incorrectly received frame before this station commences transmission.
For a network which integrates in MAC procedures to support LAN applications with quality of service (QoS) requirements, an Arbitration IFS (AIFS) is also defined. It is to be noticed that, for complying with the QoS requirements, it is introduced a coordinator that performs bandwidth management including the allocation of transmission opportunities (TXOPs) to wireless stations. The TXOP is the duration during which the TXOP holder maintains uninterrupted control of the medium, and it includes the time required to transmit frames sent as an immediate response to the TXOP holder's transmission. A TXOP may particularly comprise at least one frame and a corresponding acknowledgment. Stations compete thus on TXOPs. The owner of a TXOP has the right to transmit one or more frames during TXOP. The AIFS is a fixed time interval that starts once the medium is sensed to be idle after a correctly received TXOP frame. Once the AIFS elapsed and if the medium is still idle, the random backoff time is decremented.
An example according to IEEE 802.11-2007 standard of an access management is given with reference to FIG. 1, wherein, after a time 1 during which the medium is busy, a fixed deferral time 2 (for example a DIFS) and a random backoff time 3 are successively counted down, until the transmitting station can transmit a frame 4 on the medium still idle. Then an acknowledgement “ACK” 6 is received by the transmitting station from a receiving station, after a pause 5. Once the acknowledgement “ACK” 6 is received, the transmitting station can start again the deferral 7 and backoff 8 procedures.
Another example according to IEEE 802.11-2007 of an access management is given with reference to FIG. 2, wherein, after a time 1 during which the medium is busy, a fixed deferral time 2 (for example AIFS) and a random backoff time 3 are successively counted down, until the transmitting station can transmit one or more frames during a time called TXOP 9 on the medium still idle. The duration of the frame(s) is equal to or less than the TXOP owned by the transmitting station. It is to be noticed that, according to this particular standard, the equivalent of “ACK” acknowledgement is contained in the TXOP. Once the station has transmitted its frames or the TXOP duration 9 has expired, the transmitting station can start again deferral 7 and backoff 8 procedures.
It is to be noticed that, in the case of mesh networks according to 802.11s draft standard, wherein a particular “Mesh Deterministic Access” (MDA) is used, the TXOP is called MDA TXOP (Mesh Deterministic Access TXOP). A station that has previously reserved a MDAOP according to 802.11s dradt standard, uses the said CSMA/CA and backoff procedures according to 802.11-2007 standard to obtain a MDA TXOP.
In spite of these known access control mechanisms, medium access problems are exacerbated in systems where the node density is high and where hidden nodes exist.
A prime example where such problems likely occur is in mesh networks, such as mesh networks complying with IEEE 802.11s drafted standard.
FIG. 3 depicts such a situation where, in a mesh wireless network, each station has different neighborhood. The network topology and the environment determine the amount of neighbors per station. Here, station “A” has a single neighbor—station “B”. However, station “B” shares the wireless medium with four other stations (“A”, “C”, “D” and “E”). Due to its fewer amount of neighbors, “A” detects the wireless medium more often as idle than “B” since it is informed solely of the transmissions of its first neighbor. Thus, “A” can easily send more traffic to “B” than vice versa. Thus, station “A” can easily congest “B”.
Accordingly, due to its opportunistic medium access, the 802.11 MAC provides a station at the edge of the network with higher share of capacity.
On the contrary, stations in the center are much more polled and become more quickly bottleneck to the network, as they forward the aggregated traffic of all attached stations.
This is especially problematic for wireless mesh networks in which stations density is particularly high.
As a consequence, an edge station can easily congests its neighbors and overload them with large amount of frames.
Higher layer protocols then need to detect frame loss and thereby limit the traffic.